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Page 210 ï~~210 THE MICHIGAN BOTANIST Vol. 45 NOTEWORTHY COLLECTION MINNESOTA Pistia stratiodes L. (Araceae). Waterlettuce. Previous knowledge: Pistia stratiodes is a free-floating aquatic plant native to South America. The history of its introduction to Florida and its ecological impact there and elsewhere have been reviewed by Schmitz et al. (1993). Although it has been largely supplanted by the competitively superior water hyacinth (Eichornes crassipes), another nonnative invasive species from South America, nearly 1500 hectares of public waters in Florida were estimated to be infested by waterlettuce in 1991. Like water hyacinth, waterlettuce forms dense floating mats that shade submerged plants, reduce dissolved oxygen levels, and increase siltation. Waterlettuce is generally considered to be a tropical species. It is widely available in the United States for use in large aquaria and water gardens. The National Plant Data Center has mapped occurrences of waterlettuce along the Atlantic coast as far north as New York (USDA, NRCS 1999). Significance. Waterlettuce was found in Lake Winona, in the city of Winona, Winona County, Minnesota. The lake is divided into two basins by the Huff Street causeway. The western basin, hereafter referred to as upper Lake Winona, is approximately 1 km long, with a surface area of 36 hectares and a maximum depth of 7 m (Fremling and Heins 1986). It is fed at its western end by County Ditch 3, which carries water from the Gilmore Creek watershed. The ditch is approximately 10 meters wide where it enters the lake and is shallow and slowmoving. The eastern basin, henceforth referred to as lower Lake Winona, is approximately 2 km long, with a surface area of 93 hectares and a maximum depth of 12 m (Fremling and Heins 1986). It is connected by a culvert to the upper lake. Both basins are relatively shallow and have extensive areas vegetated by aquatic macrophytes. We first observed waterlettuce during an electrofishing survey in upper Lake Winona on 2 October 2000. At that time we estimated that several dozen plants occurred in County Ditch 3, which had no detectable flow during the time that these and subsequent observations were made. Waterlettuce plants occurred along both shorelines of the ditch within 75 m of its mouth, where the ditch is bordered to the north by Winona Senior High School and to the south by a public biking and jogging path. Relatively large and small plants in a small sample collected during electrofishing had outer leaves that measured 13-15 and 10 cm in length, respectively. Subsequent observations on 9 October and 12 October 2000 revealed no additional plants in County Ditch 3 in the 100 m reach upstream from those found earlier. However, additional plants were found around the periphery of upper Lake Winona, and a single plant was found in lower Lake Winona. Over 250
Page 211 ï~~2006 THE MICHIGAN BOTANIST 211 2006 THE MICHIGAN BOTANIST 211 FIGURE 1. Waterlettuce (Pistia stratiodes) removed from Lake Winona, Winona County, Minnesota, during October, 2000. plants were removed from the system (Figure 1). The plants appeared to be entering a senescent state. Water temperature was 120C on 9 October but was not measured on the other dates. After the Saint Mary's University (SMU) authors reported their discovery of waterlettuce in October 2000 to the Minnesota Department of Natural Resources (DNR), ensuing local publicity (Christenson 2000, Gustafson 2000) stimulated reports by Winona State University personnel that waterlettuce (1997-1999) and other nonnative invasive floating plants (E. crassipes: 1995-1996; Salvinia molesta: 1997-1998) had occurred in Lake Winona in prior years, with several hundred water lettuce plants having been removed from the western end of upper Lake Winona and County Ditch 3 in 1999 (Carol Jefferson and Neil Mundahl, personal communication). A review of previous work on the life history of waterlettuce (Datta & Biswas 1970, Dray & Center 1989, Pieterse et al. 1981) suggests that its occurrence in Lake Winona in multiple years is best explained by repeated introductions of individuals originally held in aquaria or water gardens. It is unlikely that individual plants can survive the Minnesota winter, even though aerators on Lake Winona keep small areas of its surface free of ice. Although it is conceivable that plants introduced early in any given growing season could produce seeds before the onset of cold weather in the fall, it is unlikely that seeds that have overwintered in Minnesota would germinate in time to permit the production of seeds
Page 212 ï~~212 THE MICHIGAN BOTANIST Vol. 45 during the second growing season. Pieterse et al. (1981) indicated that seeds do not germinate until temperature exceeds 200C. Concentrations of large numbers of waterlettuce plants at the western end of the upper lake indicate that waterlettuce was introduced in that vicinity, probably into County Ditch 3. Plants introduced during spring or early summer would be able to proliferate asexually in the ditch during the summer; individuals that drifted or were blown into the lake proper could be propelled by prevailing westerly winds to other portions of the shoreline. Even though we do not believe that waterlettuce will be permanently established in Minnesota, we believe that this case is noteworthy for two reasons. First, quantitative analyses of patterns in the ecology of exotic species require data on failed introductions as well as the establishment of successfully reproducing populations (Allen & Ramcharan 2001). Second, the occurrence in any area of obvious, relatively easily observed exotic species may serve as a warning that other, less readily detectable species are also being introduced; some of these may be capable of establishing persistent populations. In the present case, it is easy to imagine the possibility that other organisms (e.g., algae, snails, or fish) were intentionally or unintentionally introduced into Lake Winona when waterlettuce and other plants were released. Indeed, one of the authors (PAC) collected an adult redear slider (Trachemys scripta), a turtle native to the southern U.S., in Lake Winona in September 2002. It is fortunate that the waterlettuce and other floating plants reported here were released into the relatively confined waters of Lake Winona rather than into the nearby Mississippi River. It is possible that the river could carry floating plants far enough south that the length of growing season would no longer prevent the establishment of successfully reproducing populations. We hoped that local press releases (Christenson 2000, Gustafson 2000) would help educate the public about the danger of releasing aquarium species. No waterlettuce has been observed in Lake Winona during subsequent years, although the presence of Eurasian watermilfoil (Myriophyllum spicatum) was confirmed in 2006. Specimen citation. MIN 456176. ACKNOWLEDGMENTS We thank Dr. Matyas Buzgo of the Jodrell Laboratory, Royal Botanic Gardens, for insight into the reproductive ecology of Pistia stratiodes, Dr. Carol Jefferson and Dr. Neil Mundahl of Winona State University for sharing their observations of exotic species in Lake Winona, and Dr. Anita Cholewa of the Bell Museum of Natural History for curatorial assistance. Sam Pociask and Hannah Warthesen are grateful for Saint Mary's University's support of undergraduate research. LITERATURE CITED Allen, Y.C. & C.W. Ramcharan. 2001. Dreissena distribution in commercial waterways of the U.S.: using failed invasions to identify limiting factors. Canadian Journal of Zoology 58:898-907. Christenson, J. 2000. DNR tosses lettuce from Lake Winona. Winona Daily News, October 28, 1A. Datta, S.C. & K.K. Biswas. 1970. Germination pattern and seedling morphology of Pistia stratiodes L. Phyton 27:157-161.
Page 213 ï~~2006 THE MICHIGAN BOTANIST 213 Dray, F.A. & T.D. Center. 1989. Seed production by Pistia stratiodes in the U.S. Aquat. Bot. 33:155-160. Fremling, C.R. & G.A. Heins. 1986. A Lake Winona compendium: information concerning the reclamation of an urban winter-kill lake at Winona, Minnesota, 2nd ed. Winona State University, Winona, Minnesota. Gustafson, E. 2000. Exotic plants in Lake Winona catch DNR's attention. Winona Post, October 29. Pieterse, A.H., L. Delange, & L. Verhagen. 1981. A study on certain aspects of seed germination and growth of Pistia stratiodes. Acta Bot. Neerl. 30:47-57. Schmitz, D.C., J.D. Schardt, A.J. Leslie, F.A. Dray, Jr., J.A. Osborne, & B.V. Nelson. 1993. The ecological impact and management history of three invasive alien aquatic plant species in Florida. Pp. 173-194 in: McKnight, B.N. (ed.). Biological pollution: the control and impact of invasive exotic species. Indiana Academy of Science, Indianapolis, Indiana. United States Department of Agriculture, Natural Resources Conservation Service. 1999. The PLANTS database (http://plants.usda.gov/plants). National Plant Data Center, Baton Rouge, LA 70874-4490 USA. -Philip A. Cochran, Samuel Pociask, and Hannah Warthesen Biology Department Saint Mary's University of Minnesota 700 Terrace Heights Winona, Minnesota 55987-1399 Nick Proulx Ecological Services Minnesota Department of Natural Resources 500 Lafayette Road, Box 25 Saint Paul, MN 55155